Fever As a Symptom of Cancer

Wife taking hudband'’s temperature
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Fever is always something that should get your attention. But most of the time, it's due to an infection that can be easily managed without complication. However less commonly, fever can also be a symptom of cancer—specifically, leukemia or lymphoma.

There's a reason that your doctor doesn't jump to the possibility of cancer right away. For example, if you’re otherwise healthy, it’s in the middle of flu season, and you develop a fever along with a sore throat, nasal congestion, cough, headache, and fatigue, it's overwhelming more likely that you have influenza.

But there are times when fever, whether alone or in combination with other symptoms, is a part of something else, and your doctor is trained to pick up on clues that indicate the need for a closer look. This ties into an exploration of what's known as a "fever of unknown origin."

Remember, though, that the vast majority of fevers seen by doctors can be traced to non-malignant causes.

Fever of Unknown Origin

In the medical community, FUO stands for “fever of unknown origin.” This term is used to describe a fever that meets all of the following criteria:

  • Temperature of 101 degrees F or more
  • Lasts for at least three weeks
  • Has no other identifiable cause after three days of investigation in the hospital or after three or more outpatient visits

FUO is not just any old fever that lacks an obvious cause. In FUO, the fever has to be prolonged and there has to be a pretty extensive medical workup to rule out other numerous likely causes.

The length of time in this definition, however, conveniently tends to get rid of some of the numerous, more common causes of fever that resolve within three weeks.

Having a fever of unknown origin does not necessarily mean you have cancer. In fact, there are a huge number of possible causes that are noncancerous.

But as some people with leukemia or lymphoma can attest to, a fever that wouldn’t go away—maybe together with fatigue and a lump—was how it all began for them.

Possible Causes of FUO

Unfortunately for doctors and patients, the list of possible causes—even for fevers lasting longer than three weeks—is quite long.

Statistically, the breakdown of likely causes depends on things like your geography (where you live in the world) and your demographics (for instance, whether you are a child or a grown-up).

Among FUO causes of U.S. adults:

  • Infections: These account for about 15 percent to 25 percent of cases.
  • Diseases/conditions: Malignancies (especially blood cancers), autoimmune diseases, rheumatic diseases, suppressed immune system (as with HIV), alcoholic hepatitis, and deep vein thrombosis, for example
  • Medication use: Sometimes drugs may be to blame, including certain antibiotics, medications that are taken to prevent seizures, and even pain medications like nonsteroidal anti-inflammatory drugs (NSAIDs).

The latter two, along with a long list of miscellaneous causes, account for the remainder of those causes that are identified. Know, though, that no cause can ever be found in about 20 percent to 50 percent of FUO cases overall.

Why Blood Cancers Cause Fevers

Weight loss, fatigue, and fevers may all go together in the case of cancer, and two kinds of blood cancer in particular—lymphoma (especially non-Hodgkin) and leukemia—are known to produce fevers. These diseases, in fact, are the most common malignancies for which fever is an early sign.

While infection is always a possible cause for a fever, it’s believed that in some cases of leukemia and lymphoma, the malignant cells themselves may produce chemical signals that cause the body to elevate core temperature.

When blood cancers do cause fevers, those fevers can, in some cases, impact the stage and prognosis (or outlook) of the illness.

In some blood cancers, the presence of fever, night sweats, and unintentional weight loss suggests the cancer is more advanced and more intensive treatment might be required.

A Word From Verywell

Although blood cancers like leukemia and lymphoma do often present with a fever that won’t go away, it’s important to work with your doctor to rule out more obvious causes, such as infection. Consider your entire clinical picture with your doctor, including other signs or symptoms that could make a different diagnosis more likely.

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